The present invention relates to integrating the ignition coil driver or ignitor and associated electronics into the coil housing, and more particularly for integrating the electronics into the coil more directly by mounting the ignition insulated gate bipolar transistor and control chips directly to a printed circuit board serving as a direct connection to the connector terminals and coil leads via plated through-apertures.
Chip-on-board and surface-mount devices are types of integrated circuits well known in the art. A chip-on-board is generally an integrated circuit chip attached with an adhesive to a substrate, such as a circuit board, with wires electrically interconnecting the chip to conductors on the substrate. The wires and the interconnection are typically formed using wire bonding techniques, in which very thin electrically conductive wires, often on the order of about 0.25 millimeter or less in diameter, are bonded to leads on the chip and to bond pads on the substrate. Suitable wire bonds can be achieved with various techniques, including thermosonic and ultrasonic bonding. In contrast, surface-mount integrated circuit devices are generally characterized by being electrically and mechanically attached to the circuit board with one or more terminals or leads that are soldered, such as with a tin-lead solder, to a corresponding number of conductors on the surface of the substrate.
While wire-bonded devices are very robust, the wires are susceptible to damage during handling. Therefore, chip-on-board devices are typically encapsulated in order to protect the wires. However, suitable encapsulant materials may introduce a mismatch in coefficient of thermal expansion with the chip and substrate, which lowers the resistance of a chip-on-board to thermal cycle fatigue. As a result, chip-on-board devices are widely used in consumer electronic products having modest temperature requirements on the order of up to about 80xc2x0 Celsius, but have found limited use in more demanding environments, such as automotive and aerospace applications requiring an upper temperature capability of about 125xc2x0 Celsius, or more.
One type of known ignition circuit for switching primary current of the ignition coil is an encapsulated ignition circuit, such as an integrated circuit die chip, molded into an epoxy resin. The known encapsulated ignition circuit is buried in the potting resin of the ignition coil. Since the co-efficient of linear expansion of the potting resin of the ignition coil, approximately 30xc3x9710xe2x88x926 to 60xc3x9710xe2x88x926, is much larger than that of the components of the molded ignition circuit (approximately 3.5xc3x9710xe2x88x926 to 25xc3x9710xe2x88x926), stress is applied to circuit elements of the encapsulated ignition circuit from the potting resin due to temperature changes. Such stress can cause peeling at a bonding portion (solder bonding portion or wire bonding portion) of the circuit elements or a crack, causing potential failure of the ignition circuit. Other known encapsulated ignition circuits have been wrapped in a cushioning material, such as soft resin, and then buried in the potting resin of the ignition coil. The cushion wrapped encapsulated ignition circuits can prevent peeling at a bonding area of the circuit elements or cracking by absorbing the linear expansion difference between the potting resin and the components of the encapsulated ignition circuit with a deformation of the cushioning material, and by minimizing the stress which is applied to the circuit elements of the molded ignition circuit from the potting resin.
Current plughole coils use an over molded integrated circuit package that consists of a copper lead frame, heat sink, ceramic substrate, control chip, insulated gate bipolar transistor (IGBT), chip capacitors, and control chip. To assemble the electronics into the coil, a second stamped lead frame that has been insert molded into the coil housing is used. The electronics package is ultrasonically welded to the coil lead frame and then the coil leads are soldered to the lead frame. The electronics are ultimately packaged twice, once through the electronic supplier and a second time when the electronics are assembled into the coil.
It would be desirable to eliminate some electrical connections to increase the reliability of the electronics. It would be desirable to reduce the cost of the assembly by eliminating the packaged integrated circuit lead frame, the ceramic substrate, and by reducing the number of assembly steps. It would be desirable to reduce the amount of capital associated with a particular integrated circuit design, allowing design changes to be implemented more quickly and less costly since the electronics would not have to be constrained to fixed package sizes and fixturing for transfer molding and lead frame stamping.
The present invention includes a product, product by process, and process for integrating the ignition coil driver or ignitor and associated electronics into the coil housing. The present invention integrates the electronics into the coil more directly by mounting the ignition insulated gate bipolar transistor and control chips directly to a printed circuit board serving as a direct connection to the connector terminals and coil leads via plated through-apertures. The problems associated with heat generated in the electronics and the thermal stresses resulting from differences in the coefficient of thermal expansion can be addressed according to the present invention by protecting the electronics with at least one protective layer. The protective layer can include a conformal coating, a soft buffer material, a hard epoxy globtop, or any combination thereof, over the components and wire bonds locally on the board.
Changes to the electronic circuit according to the present invention can be made in less time and for less cost, since the components can be laid out on the printed circuit board and do not require lay out on a ceramic substrate as previously required in a standard over molded integrated circuit package. In the past, circuit design changes sometimes required retooling of the coil electronics housing due to an increase in the over molded package size.